Lithographic apparatus and device manufacturing method

ABSTRACT

An attenuation adjustment device is disclosed that includes a plurality of members configured to cast penumbras in a radiation beam illuminating a patterning device in a lithography apparatus. Furthermore, an attenuation control device may be provided to adjust the members in such a manner as to control attenuation of a radiation beam projected onto a target portion of a substrate across the cross-section of the radiation beam.

This application is a continuation-in-part application of co-pendingU.S. patent application Ser. No. 11/224,303, filed Sep. 13, 2005, whichis incorporated herein in its entirety by reference.

FIELD

The present invention relates to a lithographic apparatus and a devicemanufacturing method.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a target portion of a substrate. Lithographic apparatus can beused, for example, in the manufacture of integrated circuits (ICs). Inthat instance, a patterning device, which is alternatively referred toas a mask or a reticle, may be used to generate a circuit patterncorresponding to an individual layer of the IC, and this pattern can beimaged onto a target portion (e.g. comprising part of, one or severaldies) of a substrate (e.g. a silicon wafer) that has a layer ofradiation-sensitive material (resist). In general, a single substratewill contain a network of adjacent target portions that are successivelyexposed. Lithographic apparatus may be of the transmissive type, whereradiation is passed through a patterning device to generate the pattern,or of the reflective type, where radiation is reflected from thepatterning device to generate the pattern. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at once, andso-called scanners, in which each target portion is irradiated byscanning the pattern through the beam in a given direction (thescanning-direction) while synchronously scanning the substrate parallelor anti-parallel to this direction. It is also possible to transfer thepattern from the patterning device to the substrate by imprinting thepattern onto the substrate.

In general, there is a non-uniformity in the intensity of radiationwhich is imaged onto the substrate in such apparatus. This is typicallycaused by, for example, the mirrors or lenses of the projection systemhaving differing reflectivity or transmission over their surfaces. Inthe case of conventional lithography, so-called deep-UV (DUV), atransmissive filter is included which corrects for this non-uniformity.In the past the properties of the filter were fixed and could not bechanged over time. In newer systems the filter is adjustable, and can beadjusted to take account of slow variations in beam uniformity, forexample caused by gradual degradation of lens surfaces.

A known adjustable uniformity correction unit for DUV comprises twotransmissive plates which are considerably bigger than the beam.Different transmission profiles are provided on the plates, so that,when the transmission of the plate is to be adjusted, the point at whichthe beam intercepts the plate is changed by moving the plate. The platesare made from glass and are heavy, consequently their movement is slow.In any event, they are designed and intended to be used to correct forvery slow variations.

In extreme ultraviolet (EUV) lithography, there are no materialsavailable which can be used in a transmissive way. Accordingly anarrangement is disclosed in U.S. Pat. No. 6,741,329 in whichnon-transmissive blades, commonly called venetian blinds (‘blades’), areused to adjust the beam to correct for non-uniformity in the intensityof radiation imaged onto the substrate. In the simplest case the bladesare in the form of a series of rectangles that are rotatably mounted andare spread across the beam. In more complicated cases the blades canhave a more complicated (‘asymmetric blades’) shape. In order to reducethe beam intensity in a given location, the blade at that location isrotated so that it partially blocks the beam. The blades are typicallylocated a distance D≧B/tan(a sin(NA)) mm below the reticle where B isthe distance between the blades and NA is the numerical aperture atreticle level. If the blades were to be located closer to the reticle,then sharp images of the blade edges would appear on the substrate.Conversely, if the blades were to be moved further away from thereticle, then the spatial frequency of the intensity correction providedby the blades would be reduced.

The blade arrangement of U.S. Pat. No. 6,741,329 may not allow theuniformity or the intensity of the radiation incident on the substrateto be varied in the direction in which the substrate is scanned by thebeam during a scan. Instead the energy per laser pulse is varied duringthe scan to generate a varying intensity profile in the scanningdirection. However, unlike DUV lithography sources, EUV lithographysources are not configured to change their output power, and there istherefore no simple way in which the overall intensity of the beamincident on the substrate can be varied.

SUMMARY

An aspect of one or more embodiments of the present invention is toprovide a novel lithographic apparatus enabling control of the intensityof the beam incident on the substrate.

According to an aspect of the invention, there is provided alithographic apparatus comprising:

-   an illumination system configured to condition a radiation beam;-   a support constructed to hold a patterning device, the patterning    device being constructed to impart a cross-sectional pattern to the    radiation beam to form a patterned radiation beam;-   a substrate table constructed to hold a substrate;-   a projection system configured to project the patterned radiation    beam onto a target portion of the substrate; and-   an attenuation adjustment device comprising a plurality of members    configured to cast penumbras in the radiation beam illuminating the    patterning device, each member configured to increase or decrease    its penumbra by displacement of a portion of the member in a    direction substantially perpendicular to an axis of the member.

According to an aspect of the invention, there is provided alithographic apparatus, comprising:

-   an illumination system configured to condition a radiation beam;-   a support constructed to hold a patterning device, the patterning    device being constructed to impart a cross-sectional pattern to the    radiation beam to form a patterned radiation beam;-   a substrate table constructed to hold a substrate and move the    substrate in a scanning direction;-   a projection system configured to project the patterned radiation    beam onto a target portion of the substrate;-   an attenuation adjustment device comprising a plurality of members    configured to cast penumbras in the radiation beam illuminating the    patterning device; and-   an attenuation control device configured to adjust the members so as    to control attenuation of the radiation beam, during scanning    projection of the patterned radiation beam, in the scanning    direction across the radiation beam and in a second direction across    the radiation beam substantially perpendicular to the scanning    direction, the attenuation control device comprising a respective    position detector configured to provide an output indicative of    position of each member in dependence on detection of a beam of    detecting radiation reaching the position detector after attenuation    by the member.

According to an aspect of the invention, there is provided a devicemanufacturing method, comprising:

-   casting penumbras on a patterning device using a plurality of    members in the path of the radiation beam, each member configured to    increase or decrease its penumbra by displacement of a portion of    the member in a direction substantially perpendicular to an axis of    the member;-   imparting a cross-sectional pattern to the radiation beam using the    patterning device to form a patterned radiation beam; and-   projecting the patterned radiation beam onto a target portion of a    substrate.

According to an aspect of the present invention, there is provided alithographic apparatus comprising:

-   an illumination system configured to condition a radiation beam;-   a support constructed to hold a patterning device, the patterning    device being constructed to impart a cross-sectional pattern to the    radiation beam to form a patterned radiation beam;-   a substrate table constructed to hold a substrate; a projection    system configured to project the patterned radiation beam onto a    target portion of the substrate;-   an attenuation adjustment device comprising a plurality of members    configured to cast penumbras in the radiation beam illuminating the    patterning device; and-   an attenuation control device configured to adjust the members so as    to control attenuation of the radiation beam across the    cross-section of the radiation beam, the attenuation control device    comprising a respective position detector configured to provide an    output indicative of position of each member in dependence on    detection of a beam of detecting radiation reaching the position    detector after attenuation by the member,-   wherein the attenuation control device comprises a common radiation    source configured to generate beams of detecting radiation to detect    positions of the members.

Thus the attenuation control device, which is typically in the form of aseries of venetian blinds (the ‘blades’), can be used to correct fornon-uniformity to a high level of accuracy, and to decrease the overallintensity of the beam. This is useful because, as previously indicated,EUV lithography sources are not configured to change their output power.Because a single source is used to supply detecting radiation to all ofthe position detectors, the number of components is decreased, resultingin lower cost, more space, greater reliability and less heat generationcoupled with better possibilities for cooling. Also inaccuracies due tofluctuations or temperature variations are largely avoided, and themechanical adjustment of the arrangement becomes more straightforward.

According to a further aspect of the present invention, there isprovided a lithographic apparatus comprising:

-   an illumination system configured to condition a radiation beam;-   a support constructed to hold a patterning device, the patterning    device being constructed to impart a cross-sectional pattern to the    radiation beam to form a patterned radiation beam;-   a substrate table constructed to hold a substrate;-   a projection system configured to project the patterned radiation    beam onto a target portion of the substrate;-   an intensity adjustment device comprising a plurality of members    configured to cast penumbras in the radiation beam illuminating the    patterning device, at least one of the members having a    non-rectangular shape; and-   an attenuation control device configured to adjust the members so as    to control attenuation of the radiation beam across the    cross-section of the radiation beam.

The attenuation control device may comprise a reference detectorconfigured to provide a reference output in dependence on detection of abeam of detecting radiation reaching the reference detector directlyfrom the common radiation source. The reference detector directlydetects the radiation from the source and provides a reference outputsignal so that fluctuations of the radiation source, due to thermaldrift, for example, can be compensated in an electronic control circuit.

In some embodiments the attenuation control device comprises a mixingunit configured to receive detecting radiation from the common radiationsource and to emit a respective beam of detecting radiation through arespective aperture in the unit towards each of the members. In anembodiment, the mixing unit has reflective walls configured to multiplyreflect detecting radiation from the common radiation source towards theaperture. In this manner the characteristics of the radiation from theradiation source are scrambled due to the multiple internal reflectionswithin the mixing unit and the amount of radiation passing through eachaperture is dependent on the geometry of the mixing unit and issubstantially unaffected by the source strength or other characteristicsof the source.

The attenuation control device conveniently comprises a detection vaneportion of each member spaced from a blade part of the member configuredto cast a penumbra in the radiation beam illuminating the patterningdevice, the detection vane portion of each member configured toattenuate the beam of detecting radiation detected by the associatedposition detector. Although it is desired that the detection vaneportion is a separate portion of the member to the blade part configuredto cast a penumbra in the radiation beam, preferably being disposed on acommon shaft to the blade part, it would also be possible for thedetection vane portion to be constituted by the same part of the memberas the blade part.

In an embodiment, the apparatus includes a scanning system configured toprovide relative movement between the radiation beam and the targetportion of the substrate in a scanning direction, the members beingdistributed along a path transverse to the scanning direction. In thiscase the scanning system may comprise a curved slit extending along thepath through which the radiation beam is projected onto the targetportion of the substrate, and the attenuation control device may beadapted to adjust the members by different amounts in such a manner thatthe intensity of the radiation beam is substantially constant over thelength of the slit.

The attenuation control device may be arranged to adjust the members insuch a manner as to permit the intensity of the radiation beam projectedonto the target portion of the substrate to be varied both in thescanning direction and in a direction transverse to the scanningdirection.

Furthermore the attenuation control device may be configured to usefeedback control to supply a control signals to at least one of themembers to drive that member to the an adjustment position according tothe output indicative of the position of that member received from theposition detector.

The members of the attenuation control device are typically a series ofblades that are tiltable about tilt axes so as to adjust the widths ofthe penumbras that they cast and are disposed with their tilt axessubstantially parallel to one another. In the case in which the membersof the attenuation control device are in the form of venetian blinds,since the blades are very small and light, they may be rotated quicklyand therefore may be used to provide real time uniformity correction. Ingeneral, during exposure of a target (die), radiation is scanned acrossthe target in the Y direction. In a prior art arrangement, thenon-uniformity previously measured in the Y direction could be correctedfor by adjusting the intensity of the illumination provided by the DUVlaser (laser pulse energy control). However, in an embodiment of thepresent invention, the venetian blinds blades are used in real time toadjust the uniformity. For example, during exposure of a target, theblades may be progressively rotated to compensate for a previouslymeasured ramp in the exposure intensity (as previously mentioned, theintensity of the radiation generated by the EUV source cannot beadjusted). In addition to this, the blades may be adjusted in advance totake account of variation across the X direction, and are fixed duringthe scan. In an alternative or additional embodiment the positions ofthe blades may be varied during the scan to take account of variation inthe X direction during the scan.

In general it is desired to make the intensity of the radiation incidenton the substrate as uniform as possible, and to keep the same uniformintensity across the entire substrate. However, other processes that areoutside the user's control, such as chemical processing of thesubstrate, may have an effect which varies for different locations onthe substrate. Typically, there may be a difference between the centerof the substrate and the edge of the substrate. An embodiment of thepresent invention allows for effect of these processes to be measuredand then for the intensity of the beam to be adjusted to correct forthis. For example, the intensity of illumination at the edge of thesubstrate may be controlled to be greater than the intensity of theillumination at the center of the substrate, to take account ofdifferences in processing that occur at the edge of the substratecompared to the center of the substrate.

However, what would be even more useful would be to be able to adjust inthe Y direction and also in the X direction. An embodiment of thepresent invention allows this to be done. Using an embodiment of theinvention, specific areas of the die may be given, for example, a lowerdose than other areas of the die.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts and inwhich:

FIG. 1 diagrammatically shows a lithographic apparatus having areflective patterning device;

FIG. 2 is a perspective view of an attenuation control device;

FIG. 3 is a diagram of a control arrangement for controlling thepositions of the blades of an attenuation control device used in alithographic apparatus in an embodiment of the invention;

FIG. 4 shows a detail of the attenuation control device;

FIG. 5 is an explanatory diagram indicating the difference in theradiation received by the angle detector depending on the angle of theblade;

FIG. 6 is a diagram showing parts of the control arrangement of theattenuation control device;

FIG. 7 shows several shapes of attenuation blade: (a) rectangular(symmetric), (b) asymmetric, (c) partially transmissive, (d) compound,and (e) 3-dimensional; and

FIG. 8 illustrates an alternate embodiment of the present invention;

FIGS. 9(a) and (c) are top views of an attenuation structure of anattenuation control device in respectively two different operationalconditions;

FIGS. 9(b) and (d) are respective side views of the attenuationstructure depicted in FIGS. 9(a) and (c);

FIG. 10 is a top view of an embodiment of an attenuation control device,comprising a plurality of attenuation structures, positioned in relationto an illumination field; and

FIG. 11 is a top view of another embodiment of an attenuation controldevice, comprising a plurality of attenuation structures, positioned inrelation to an illumination field.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and will herein be described in detail. Itshould be understood, however, that this specification is not intendedto limit the invention to the particular forms disclosed herein, but onthe contrary, the intention is to cover all modifications, equivalents,and alternatives falling within the scope of the invention, as definedby the appended claims.

DETAILED DESCRIPTION

FIG. 1 schematically depict an example of a lithographic apparatus. Theapparatus includes:

an illumination system (illuminator) IL configured to provide a beam PBof radiation (e.g. UV radiation);

a support structure (e.g. a mask table) MT configured to hold apatterning device (e.g. a mask) MA and connected to first positioner PMconfigured to accurately position the patterning device with respect toitem PL;

a substrate table (e.g. a wafer table) WT configured to hold a substrate(e.g. a resist-coated wafer) W and connected to a second positioner PWconfigured to accurately position the substrate with respect to item PL;and

a projection system (e.g. a refractive or reflective projection lens) PLconfigured to image a pattern imparted to the beam PB by the patterningdevice MA onto a target portion C (e.g. comprising one or more dies) ofthe substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure holds the patterning device in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device”.

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a reflective type (e.g. employinga programmable mirror array of a type as referred to above, or employinga reflective mask). Alternatively, the apparatus may be of atransmissive type.

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more support structures). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser, asmay be the case for transmissive apparatus. In such cases, the source isnot considered to form part of the lithographic apparatus and theradiation beam is passed from the source SO to the illuminator IL withthe aid of a beam delivery system BD comprising, for example, suitabledirecting mirrors and/or a beam expander. In other cases the source maybe an integral part of the lithographic apparatus, for example when thesource is a mercury lamp. The source SO and the illuminator IL, togetherwith the beam delivery system BD if required, may be referred to as aradiation system.

The illuminator IL may comprise an adjuster for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator and acondenser. The illuminator provides a conditioned beam of radiation,referred to as the beam PB, having a desired uniformity and intensitydistribution in its cross-section.

The radiation beam PB is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., mask table) MT, and ispatterned by the patterning device. Having traversed the patterningdevice MA, the radiation beam PB passes through the projection systemPS, which focuses the beam onto a target portion C of the substrate W.With the aid of the second positioner PW and position sensor IF (e.g. aninterferometric device, linear encoder or capacitive sensor), thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the radiation beam PB.Similarly, the first positioner PM and another position sensor (which isnot explicitly depicted in FIG. 1) can be used to accurately positionthe patterning device MA with respect to the path of the radiation beamPB, e.g. after mechanical retrieval from a mask library, or during ascan. In general, movement of the support structure MT may be realizedwith the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which form part of the firstpositioner PM. Similarly, movement of the substrate table WT may berealized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the support structure MT may be connected to ashort-stroke actuator only, or may be fixed. Patterning device MA andsubstrate W may be aligned using patterning device alignment marks M1,M2 and substrate alignment marks P1, P2. Although the substratealignment marks as illustrated occupy dedicated target portions, theymay be located in spaces between target portions (these are known asscribe-lane alignment marks). Similarly, in situations in which morethan one die is provided on the patterning device MA, the patterningdevice alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the support structure MT and the substrate table WT arekept essentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the support structure MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the supportstructure MT may be determined by the (de-)magnification and imagereversal characteristics of the projection system PS. In scan mode, themaximum size of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the support structure MT is kept essentiallystationary holding a programmable patterning device, and the substratetable WT is moved or scanned while a pattern imparted to the radiationbeam is projected onto a target portion C. In this mode, generally apulsed radiation source is employed and the programmable patterningdevice is updated as required after each movement of the substrate tableWT or in between successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

In scan mode, the support structure MT is movable in a given direction(the so-called “scan direction”, e.g., the Y direction) with a speed v,so that the beam PB is caused to scan over a patterning device image.Concurrently, the substrate table WT is simultaneously moved in the sameor opposite direction at a speed V=Mv, in which M is the magnificationof the projection system PL (typically, M=¼ or ⅕). In this manner, arelatively large target portion C can be exposed, without having tocompromise on resolution.

As illustrated in FIGS. 2 and 4, an attenuation control device 10 maycomprise a plurality of blades 11 disposed in the illumination system ILin the path of the beam PB. The attenuation control device 10 issituated at an optical distance d from the patterning device MA, or aplane conjugate with the patterning device MA, such that the bladeswould be out of focus at patterning device level and also not in a pupilplane of the illumination system. In general, the attenuation controldevice should be closer to the patterning device, or a conjugate planethereof, than to a pupil plane. If the illumination system contains anintermediate image plane, the blades may be positioned closer to thatthan to a pupil plane. In an illumination system utilizing field andpupil facet mirrors to provide uniformity, the attenuation controldevice may be positioned after the field facet mirrors.

The blades 11 extend partially or wholly across the beam so that theirhalf shadows extend partially or wholly across the width of theillumination field IFL (along the scanning direction of the apparatus),substantially perpendicular to its longitudinal axis. Usually the bladesextend over the whole slit but, when there is a strong telecentricitygradient near the edges of the slit, the edge area is desirably notblocked. The blades are spaced apart a distance such that their halfshadows at patterning device level are overlapping (though it may besufficient that they are adjacent) and must be sufficient in number sothat their half shadows cover the entire illumination field. The shadowprofiles of the blades tail-off and the tail portions overlap. Rotatingthe blades, to increase their effective widths, darkens their shadowprofiles. Actuators 12 are positioned to selectively rotate thecorresponding blades 11.

The illumination slit that is used to expose the substrate duringscanning is usually curved, as shown in FIG. 2. The blades 11 areoriented with a fixed angle of typically around 60 degrees with respectto the non-scanning direction or x-axis. Due to this angle the shadow ofeach blade is beneficially spread out in the non-scanning x-direction.Since the overlap of the blades with the slit is different on the lefthand side to the right hand side of the slit, the transmittance of theblades is not the same. The blade on the left hand side of FIG. 2 ismore nearly perpendicular to the slit, leading to a narrower spatialprofile with a relatively low transmission, whereas the blade on theright hand side of FIG. 2 is more nearly parallel to the slit, leadingto a broader spatial profile with a relatively high transmission. Byrotating the blades progressively less from left to right with respectto the x-axis, their peak transmission can be made the same. Forexample, at the left hand end of the slit the blades may be mounted at60° relative to the x-axis, whereas at the right hand side of the slitthe blades may be mounted at 45° relative to the x-axis.

Rotation of one of the blades 11 from the maximally open position shownin FIG. 5(c) causes its effective width in the beam to increase, therebyblocking a greater portion of the incident radiation. In an embodiment,the blades are made of a material absorbent of the radiation of the beamso as to minimize scattered stray radiation (or have an anti-reflectioncoating). Accordingly, the angle of inclination of individual ones ofthe blades 11 can be adjusted to absorb a greater portion of incidentradiation in regions of the beam where the incident intensity is higherso as to increase uniformity of illumination. The angle of the bladescan be varied to reduce the intensity in the half shadow by up to about10% without unduly affecting telecentricity. For a blade disposed at 90mm from the patterning device in an apparatus with NA=0.25 and σ=0.5using EUV radiation, the radius of the half shadow at patterning devicelevel is 3 mm so that about 35 blades would be used to cover anillumination field of length 104 mm, for example. Where the blades aremounted at an angle of typically 60 degrees to the scanning direction,even less blades, i.e. 23 blades, are needed to cover the slit. Inanother apparatus, e.g. using DUV radiation, the stand-off distance maybe a factor of 4 or 5 less.

Referring to FIG. 4, each of the blades 11 is made from molybdenum, is10 mm long, 2 mm wide and 0.2 mm thick and is mounted on a rotatableshaft 13. Connected to the shaft 13 of each blade 11 is a vane 14 to beilluminated by radiation (e.g., visible light) from a radiation sourceand having a radiation detector 16 located adjacent it. This is used tomeasure the orientation of the blade 11 by detecting the quantity ofradiation reaching the detector 16 from the radiation source which isdependent on the orientation of the intervening vane 14. The shaft 13 isconnected to a moving magnet 17 surrounded by a yoke 18 and coil 19 of amotor that is used to rotate the blade 11. One end of the collection ofelements is fixed to a mounting, thereby acting as a torsion bar, andthe other end is mounted in ruby bearings (not shown).

The distribution of the intensity across the slit can determine thedesired shape of the attenuation blades. In FIG. 7, a variety of bladesis shown which results in a good uniformity at substrate level,depending on the radiation distribution at the working level of theattenuation control device. In general the rectangular blades in FIG.7(a) are desired when the intensity distribution across the slit has theshape of a top hat or has linear ramps. When the intensity distributionis more Gaussian—like, an asymmetric blade, as shown in FIG. 7(b),provides the flattest intensity profile at substrate level. If the beamhas strongly non-telecentric edges, the blades desirably are shorter sothat that area is not attenuated. If the telecentricity is worst at thecenter or if for other reasons the center of the beam must not bedisturbed, a holed blade, as shown in FIG. 7(c), may be advantageous. Byplacing multiple blades, e.g. 2 blades, as shown in FIG. 7(d), on acommon rotation axis, widely different attenuation profiles can begenerated as a function of rotation angle. The same applies to the other3-dimensional shape, as shown in FIG. 7(e), in this case a helicoid, inthat here also the position of the center of gravity of the attenuationprofile can be moved by rotating the blade.

In FIG. 8 the beam PB is perpendicular to the page, its intensitycross-section is Gaussian shaped. The attenuation blades 11 numbered nand n+1 cast penumbras Pn and Pn+1 at patterning device level. As shownthe rectangular blades n and n+1 can be rotated so that the integrated(attenuated) intensities at positions xn and xn+1 are identical. Inbetween the blades however there is still a residual non-uniformity dueto the non-linearity of the PB cross section profile. By deviating fromthe rectangular blade shape, i.e. by locally widening the blade n, n+1or both (the dashed lines on the blades) the non-uniformity between theblades can be minimized.

It will also be appreciated that the exact shape of the blades is notcrucial to an embodiment of the invention, although the blades should bemade as thin as possible to provide minimum obscuration at theirmaximally open position. The width of the blades should be determined inaccordance with the accuracy of the actuators 12 to provide the desireddegree of controllability over the amount of radiation absorbed.

The actuators 12 may be, for example, piezoelectric actuators or anyother suitable rotary actuator. A linear actuator driving the rods via agear arrangement is also possible.

In an embodiment, as illustrated in FIGS. 9-11, an attenuation controldevice 10 comprises a plurality of attenuation structures 21 disposed inthe illumination system IL in the path of the beam PB. The attenuationcontrol device 10 is situated at an optical distance from the patterningdevice MA, or a plane conjugate with the patterning device MA, such thatthe attenuation structures would be out of focus at patterning devicelevel and also not in a pupil plane of the illumination system. Ingeneral, the attenuation control device should be closer to thepatterning device, or a conjugate plane thereof, than to a pupil plane.If the illumination system contains an intermediate image plane, theattenuation structures may be positioned closer to that than to a pupilplane. In an illumination system utilizing field and pupil facet mirrorsto provide uniformity, the attenuation control device may be positionedafter the field facet mirrors.

Referring to FIG. 9(a), an attenuation structure 21 comprises a centralwire 20 with at least one tube 22, 24 mounted thereon. The tube 22, 24is movable along the wire 20 by means of, for example, an actuator. Thewire 20 may be circular or any other shape and may be flexible or rigid.In an embodiment, there is only one tube 22 mounted on the wire 20. Inan embodiment, there are a plurality of tubes 22, 24 mounted on the wire20, one tube 22 of which is movable along the wire and another tube 24of which is fixed. Connected to the tube 22, 24 is a flexible material26, which, in an embodiment, comprises a plurality of small flexiblewires. Where there is only one tube 22, the flexible material 26 isconnected at one end to the tube 22 and fixed at another end to the wire20 or other structure. Where a plurality of tubes 22, 24 are provided,the flexible material 26 is connected at one end to a tube 22 of theplurality of tubes and at another end to another tube 24 of theplurality of tubes.

Referring to FIG. 9(b), in an embodiment, the tube 22, 24 completelysurrounds the wire 20 although it need not if otherwise moves in adirection along the wire 20. Similarly, in an embodiment, the flexiblematerial 26 completely surrounds the wire 20 although it need not if, asdiscussed below, the material is sufficient to be displaced to attenuateradiation.

Referring to FIG. 9(c), operation of the attenuation structure 11 toattenuate radiation according to an embodiment involves displacing bythe tube 22, 24 to cause the flexible material 26 to be displaced in adirection (X-direction) substantially perpendicular to an axis (Y-axis)of the wire 20. In an embodiment where only tube 22 is movable, thentube 22 is moved towards the end of the flexible material that isaffixed or otherwise stationary. The movement of tube 22 causes theflexible material 26 to compress and move outward from the wire 20 asshown in FIG. 9(c). In an embodiment where tubes 22, 24 are movable, onetube 22 or 24 may be moved or both tubes 22 and 24 may be moved. Forexample, tube 22 may be moved toward tube 24 or tubes 22 and 24 may bemoved toward each other, in each case causing the flexible material 26to move outward from the wire 20. FIG. 9(d) shows, for example, thedisplacement of the flexible material 26 when tube 22 is moved towardstube 24. As will be apparent, tube 22, 24 may be moved away along thewire 20 to cause the flexible material 26 to be displaced towards thewire 20.

Depending on the amount of movement of tube 22, 24, the flexiblematerial 26 can be variably displaced inward and outward from the wire20 to control the amount of attenuation effected by the attenuationstructure 21 in the X-direction and to some extent in the Y-direction. Acontroller as discussed above may be used to control the amount ofattenuation provided by the attenuation structure 21.

In an embodiment, where, for example, a plurality of movable tubes 22,24 are provided, the plurality of tubes 22, 24 may be moved,simultaneously or not, in the same direction to shift the flexiblematerial 26 along the wire 20. Thus, not only may attenuation becontrolled in a direction (X-direction) substantially perpendicular tothe axis of the wire 20, it may also be effectively controlled in adirection (Y-direction) substantially parallel to the axis of the wire20. Thus, attenuation may be effectively variably controlled in a X-Yplane.

Referring to FIG. 10, an embodiment of an attenuation control device 10comprising a plurality of attenuation structures 21 is depicted inrelation to an illumination field IFL of a lithographic apparatus. Inthis embodiment, only tube 22 of each attenuation structure 21 ismovable along the wire 20 of each attenuation structure 21 so as tocause the flexible material 26 to be displaced and thus controlattenuation effected by that attenuation structure 21. Each of theattenuation structures 21 can be individually controlled to vary theamount and spatial position of the attenuation provided by thatattenuation structure 21. In this embodiment, the attenuation isprimarily controlled in the X-direction through movement of the flexiblematerial 26 inwards or outwards from the respective wire 20 of eachattenuation structure 21.

Referring to FIG. 11, an additional or alternative embodiment of anattenuation control device 10 comprising a plurality of attenuationstructures 21 is depicted in relation to an illumination field IFL of alithographic apparatus. In this embodiment, both tubes 22 and 24 of eachattenuation structure 21 is movable along the wire 20 of eachattenuation structure 21 so as to cause the flexible material 26 to bedisplaced and thus control attenuation effected by that attenuationstructure 21. Each of the attenuation structures 21 can be individuallycontrolled to vary the amount and spatial position of the attenuationprovided by that attenuation structure 21. In this embodiment, theattenuation can be primarily controlled in the X and Y-directionsthrough movement of the flexible material 26 inwards or outwards fromthe respective wire 20 of each attenuation structure 21 and throughmovement of the flexible material 26 along the respective wire 20 ofeach attenuation structure 21.

As will be apparent, a plurality of attenuation structures 21 may beprovided wherein some of the attenuation structures 21 comprise onlytube 22 of each such attenuation structure 21 being movable along thewire 20 of such attenuation structure 21 and some of the attenuationstructures comprise both tubes 22 and 24 of each such attenuationstructure 21 being movable along the wire 20 of such attenuationstructure 21.

In an embodiment, attenuation may be completely customized in the X-Yplane of the illumination field IFL to control, for example, uniformityof the distribution of radiation. For example, the flexible material 26may be moved in the X and Y directions and selective attenuationstructures 21 may have the flexible material variably adjusted in the Xand Y directions so that attenuation of radiation in throughout X-Yplane of illumination field IFL may be variably controlled.

The attenuation structures 21 may have some or many of the features ofthe blades 11 and/or actuators 12 described above with reference FIGS.2-5 and 7-8.

Further, measurement may be provided to control the amount ofattenuation provided by the blades 11 and/or attenuation structures 21.While the following will discuss measurement and/or control in relationto blades 11, the same or similar principles may be applied toattenuation structures 21.

To detect the positions of the blades for the purpose of controlling theamount of attenuation applied by the blades, each blade may have anassociated position detector configured to detect a quantity ofradiation received from a radiation source providing a radiation beamthat is arranged to be interrupted by a portion of the blade, or anelement connected to the blade so as to rotate with the blade in such amanner that the quantity of radiation reaching the position detector isindicative of the orientation of the blade. The outputs of the positiondetectors can then be supplied to an electronic controller to controlthe actuators, used to tilt the blades, in such a manner as toaccurately orient the blades according to the degree of attenuationdesired. Generally the number of radiation sources used in such aposition detection arrangement will correspond to the number of bladeswhose positions are to be detected. Thus, if in a typical arrangement 30blades are provided, the position detection arrangement may include, forexample, 30 light-emitting diodes to emit light and 30 photodiodes todetect the light after attenuation by the blades. All the components maybe disposed in a vacuum so that, because of the lack of convection,cooling can present a problem.

When a high measurement accuracy is required, the use of multipleradiation sources can be disadvantageous in that the intensity ofradiation emitted can vary from source to source and with time dependingon the different thermal behavior of each source, which may cause therelationship between the proportion of radiation received by eachdetector and the precise orientation of the blade, as well as theangular distribution of the radiation and the degree of self-heating, tovary from source to source. The thermal drift of the light emittingdiodes can also render these unsuitable for use in high measurementaccuracy system. The use of multiple radiation sources is alsodisadvantageous in so far as it requires use of a high level ofcomponents and cabling, as well as providing high power consumption andcooling requirements.

As shown in the three explanatory diagrams of FIG. 5, the attenuationcontrol device comprises a respective radiation detector 16 configuredto provide an electrical output signal indicative of the orientation ofeach blade in dependence on detection of a beam of detecting radiationemitted by a light-emitting diode, for example, reaching the radiationdetector 16 after attenuation by a vane 14 that may be constituted bythe blade 11 itself or may be a separate part mounted on the same shaftas the blade. The diagrams (a), (b) and (c) show how the quantity ofradiation received by the detector 16 is a function of the orientationof the vane 14, although it should be appreciated that these diagramsexaggerate the range of rotation for the purposes of illustration andthat the full range of rotation is more likely to be in the range of40°, rather than 90° as shown.

In one operational mode, the attenuation control device 10 is used tocorrect for undesired non-uniformities in the beam provided by theillumination system. When used in this way, such uniformities can bemeasured by an appropriate sensor or by calibration runs. Theappropriate blade angles to achieve the desired uniformity correctionare then calculated and the actuators 12 controlled to effect this by acontroller 30 (see FIG. 3). The uniformity of the beam is thenre-measured at appropriate intervals to detect any time varyingnon-uniformities and the blade angles adjusted as necessary. For thisfunction the speed of response of the blade actuators is not crucial butthe actuators should desirably be designed so that the blade positionscan be maintained for relatively long periods without the need forconstant energization of the actuators.

In another operational mode of the attenuation control device, theblades are positioned both to correct for non-uniformity and to decreasethe intensity of the beam. This is particularly useful as the EUV sourcehas no capability to vary the pulse energy over a large range (unlikeDUV lasers). The uniformity of illumination is optimized by finding thepoint with minimum intensity and, by suitable adjustment of the blades,then cutting off all radiation above that minimum for all otherpositions in the slit through which the beam passes to the substrate,the excess radiation being absorbed by the blades. A similar uniformityprofile, but of lower intensity, may be obtained by cutting off allradiation above a lower intensity by suitable adjustment of the blades.

In a further operational mode of the attenuation control device, theadjustment of the lightweight blades performs uniformity correction (indirection X) and variable attenuation (in direction Y) simultaneouslyduring scanning of a die by the beam through a scanning slit. In thesimplest mode of operation the blades are adjusted in advance to takeaccount of variation along the length of the slit (the X direction), andthen remain fixed in these positions during scanning across the die.

In an alternative mode of operation the positions of the blades areadjusted during the scanning operation to take account of uniformityvariation in the X direction during the scan.

In general it is useful to make the illumination of the substrate asuniform as possible and to keep the same uniform intensity over theentire substrate. However other processes outside the user's control,such as the chemical processing of the substrate for example, may havean effect which varies for different locations on the substrate.Typically there may be a difference between the center of the substrateand the edge of the substrate, and in this case it is possible tomeasure the effect of such processes and to then adjust the intensity ofthe beam to correct for this effect. For example, the intensity ofillumination at the edge of the substrate may be controlled to begreater than the intensity of the illumination at the center of thesubstrate, to take account of differences in processing that occur atthe edge of the substrate as compared to the center.

Alternatively the attenuation control device may be used as atwo-dimensional attenuation controller. In this case the requirement isthat the blades can be rotated fast, e.g., within the exposure time of adie so that there can be different corrections within a die. With thecurrent concept of very lightweight blades this is possible, the fullrange of angles (40 degrees: from −5 to plus 35 degrees) can betravelled within 0.2 sec.

The control of the attenuation control device is effected by means of a‘closed-loop’ control (feedback control) arrangement in which there istwo-way communication between a controller 30 and the attenuationcontrol device 10 as shown in FIG. 3 in that a respective feedbacksignal is sent back to the controller 30 from an angle detector 16indicative of the orientation of each blade 11 of the attenuationcontrol device . The feedback signal from each angle detector 16dependent on the position of the corresponding blade 11 is transmittedthrough an amplifier 31 and an input of an adder 32 to the other inputof which a set point signal is applied, and the controller 30 sends outan adjustment signal to a corresponding actuator 12 by way of a furtheramplifier 33 if the orientation of the blade is incorrect. At each pointin time the actual angles of all the blades are known to the controller30. This method is very accurate but places a heavy burden ondata-transport and on the calculation capabilities.

As shown diagrammatically on the right hand side of FIG. 6, theradiation supplied to the vanes 14 for the purpose of detecting theorientation of the vanes 14 is supplied from a common radiation source20, such as a LED and optionally by way of an optical fiber if the LEDis positioned remotely, by way of an optical integrator 21 constitutinga mixing unit having diffusely reflective internal surfaces configuredto mix the radiation from the common source 20. The mixed radiation isemitted from the optical integrator 21 through a respective aperture 22towards each of the vanes 14. The row of apertures 22 follows the curvedshape of the slit, and the radiation integrator itself can have anyouter shape provided that all apertures 22 receive the same amount ofrandomized radiation. As previously described the quantity of radiationreaching the angle detector 16 is dependent on the orientation of thevane 14 as shown diagrammatically in FIG. 6. The optical integrator 21has reflective walls to multiply reflect radiation from the commonsource 20 towards the apertures 22, as shown diagrammatically in FIG. 6,and provides the required high homogeneity and stability with respect todrift over time and temperature to assure stable reproducible anglemeasurement. The attenuation control device also comprises a referencedetector 23 configured to provide a reference output signal independence on detection of a beam of detecting radiation reaching thereference detector directly from the common radiation source. Thereference detector directly detects the radiation from the source andprovides a reference output signal so that fluctuations of the radiationsource, due to thermal drift, for example, can be compensated in anelectronic control circuit.

As noted above, the above described measurement and control concepts maybe applied to the attenuation structures 21. So, for example, the blade11 may correspond to the flexible material 26 and angles of the blade 11may correspond to displacement of the flexible material 27.

As an example, the flexible material 26, the tube 22, 24, or some otherpart moveable with the tube 22, 24 or the flexible material 26 may beequivalent to the vane 14, i.e., the movement thereof is used toattenuate a detecting beam of radiation and the position detector isconfigured to provide an output indicative of a position of theattenuation structure in dependence on detection of the beam ofdetecting radiation reaching the position detector after attenuation.

Alternatively or additionally, a beam of detecting radiation may beredirected by the flexible material 26, the tube 22, 24, or some otherpart (e.g., a mirror) moveable with the tube 22, 24 to a detector, thedetector providing an output indicative of a position of the attenuationstructure in dependence on detection of the beam of detecting radiationreaching the position detector after redirection. For example, aninterferometer or encoder system may be used for this purpose.Alternatively or additionally, any other measurement apparatus may beused to determine the displacement of the flexible material 26 in Xand/or Y directions.

In each case, with the detected position of the flexible material 26,attenuation of the radiation beam can be controlled by adjusting theattenuation structures 21. For example, the appropriate tube 22, 24displacement to achieve the desired uniformity correction can becalculated and the actuator for the tube 22, 24 is then controlled toeffect this.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,liquid-crystal displays (LCDs), thin-film magnetic heads, etc. Theskilled person will appreciate that, in the context of such alternativeapplications, any use of the terms “wafer” or “die” herein may beconsidered as synonymous with the more general terms “substrate” or“target portion”, respectively. The substrate referred to herein may beprocessed, before or after exposure, in for example a track (a tool thattypically applies a layer of resist to a substrate and develops theexposed resist) or a metrology or inspection tool. Where applicable, thedisclosure herein may be applied to such and other substrate processingtools. Further, the substrate may be processed more than once, forexample in order to create a multi-layer IC, so that the term substrateused herein may also refer to a substrate that already contains multipleprocessed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of 365, 248, 193, 157 or 126 nm) and extremeultra-violet (EUV) radiation (e.g. having a wavelength in the range of5-20 nm), as well as particle beams, such as ion beams or electronbeams.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The description is not intended to limit theinvention.

1. A lithographic apparatus, comprising: an illumination system configured to condition a radiation beam; a support constructed to hold a patterning device, the patterning device being constructed to impart a cross-sectional pattern to the radiation beam to form a patterned radiation beam; a substrate table constructed to hold a substrate; a projection system configured to project the patterned radiation beam onto a target portion of the substrate; and an attenuation adjustment device comprising a plurality of members configured to cast penumbras in the radiation beam illuminating the patterning device, each member configured to increase or decrease its penumbra by displacement of a portion of the member in a direction substantially perpendicular to an axis of the member.
 2. The apparatus of claim 1, wherein each member comprises a central structure along the axis, a first structure movable along the central structure, and a flexible material attached to the first structure, wherein the portion of the member comprises the flexible material and movement of the first structure causes displacement of the flexible material in the direction.
 3. The apparatus of claim 2, wherein the first structure is attached to one end of the flexible material and another end of the flexible material is fixed relative the central structure at a position displaced from the first structure, the flexible material extending between the first structure and the position along at least a portion of the central structure.
 4. The apparatus of claim 2, wherein the first structure is attached to one end of the flexible material and further comprising a second structure movable along the central member to which another end of the flexible material is attached.
 5. The apparatus of claim 4, wherein both the first structure and the second structure are movable in a same direction to displace the portion of the member in a direction substantially parallel to the axis.
 6. The apparatus of claim 4, wherein the first structure and the second structure are movable towards each other to cause displacement of the flexible material in the direction.
 7. The apparatus of claim 1, wherein at least one of the plurality of members is further configured displace the portion of the member in a direction substantially parallel to the axis of the member.
 8. The apparatus of claim 1, further comprising a scanning system configured to provide relative movement between the radiation beam and the target portion of the substrate in a scanning direction, the members being distributed along a path transverse to the scanning direction.
 9. The apparatus of claim 8, wherein the scanning system comprises a slit extending across the path through which the radiation beam is to be projected onto the target portion of the substrate, and further comprising an attenuation control device arranged to adjust the members by different amounts in such a manner that an intensity of the radiation beam is substantially constant over a length of the slit.
 10. The apparatus of claim 8, further comprising an attenuation control device arranged to adjust the members in such a manner as to permit an intensity of the radiation beam projected onto the target portion of the substrate to be varied in a direction transverse to the scanning direction during the scanning.
 11. The apparatus of claim 8, further comprising an attenuation control device arranged to adjust the members in such a manner as to permit an intensity of the radiation beam projected onto the target portion of the substrate to be varied both in the scanning direction and in a direction transverse to the scanning direction.
 12. The apparatus of claim 1, further comprising a position detector configured to provide an output indicative of a position of each member in dependence on detection of a beam of detecting radiation reaching the position detector after attenuation or redirection by the member.
 13. The apparatus of claim 12, further comprising an attenuation control device configured to use feedback control to supply a control signal to at least one of the members to drive that member to an adjustment position according to the output indicative of the position of that member received from the position detector.
 14. The apparatus of claim 12, further comprising a detection vane portion of each member spaced from the portion of the member, the detection vane portion of each member configured to attenuate the beam of detecting radiation detected by the position detector.
 15. A lithographic apparatus, comprising: an illumination system configured to condition a radiation beam; a support constructed to hold a patterning device, the patterning device being constructed to impart a cross-sectional pattern to the radiation beam to form a patterned radiation beam; a substrate table constructed to hold a substrate and move the substrate in a scanning direction; a projection system configured to project the patterned radiation beam onto a target portion of the substrate; an attenuation adjustment device comprising a plurality of members configured to cast penumbras in the radiation beam illuminating the patterning device; and an attenuation control device configured to adjust the members so as to control attenuation of the radiation beam, during scanning projection of the patterned radiation beam, in the scanning direction across the radiation beam and in a second direction across the radiation beam substantially perpendicular to the scanning direction, the attenuation control device comprising a respective position detector configured to provide an output indicative of position of each member in dependence on detection of a beam of detecting radiation reaching the position detector after attenuation by the member.
 16. The apparatus of claim 15, wherein each member is configured to increase or decrease its penumbra by displacement of a portion of the member in a direction substantially perpendicular to an axis of the member.
 17. The apparatus of claim 16, wherein at least one of the plurality of members is further configured displace the portion of the member in a direction substantially parallel to the axis of the member.
 18. The apparatus of claim 15, further comprising a slit extending across the path through which the radiation beam is to be projected onto the target portion of the substrate, and wherein the attenuation control device is arranged to adjust the members by different amounts in such a manner that an intensity of the radiation beam is substantially constant over a length of the slit.
 19. A device manufacturing method, comprising: casting penumbras on a patterning device using a plurality of members in the path of the radiation beam, each member configured to increase or decrease its penumbra by displacement of a portion of the member in a direction substantially perpendicular to an axis of the member; imparting a cross-sectional pattern to the radiation beam using the patterning device to form a patterned radiation beam; and projecting the patterned radiation beam onto a target portion of a substrate.
 20. The method of claim 19, wherein at least one of the plurality of members is further configured displace the portion of the member in a direction substantially parallel to the axis of the member. 